With the recent technological advances in regard to integration and miniaturization, and by virtue of the development of low bitrate and very low consumption wireless communication technologies, a new application field has emerged by the name of wireless body networks or BAN, the acronym standing for “Body Area Networks”. Hereinafter the terms “body network” and “BAN” will be employed interchangeably. In this type of application, wireless transmission and/or reception elements are intended to form a network on or in very close proximity to a human body. Known applications of these networks are notably the “exploded terminal” (comprising screens, keyboards, and earpieces, not co-located), sports apparatus (cardio-frequency meter, watch, pedometer on a shoe) or medical apparatus (nomadic monitoring of cardiac, cerebral and muscular activity, for example).
The locating of the wireless devices constituting a BAN is of interest in numerous applications such as:                navigation of groups of people moving around indoors and geo-located services;        motion capture, for example, for the tracking of sports movements or for entertainment and games applications;        posture detection, for example, for rehabilitation, the tracking of vulnerable or elderly people, and the surveillance of people operating in a disaster-stricken environment, for example firemen operating inside a blazing building.        
Radiolocation has been studied in the context of wireless personal networks or WPANs (“Wireless Personal Area Networks”) and networks of sensors or WSN (“Wireless Sensor Networks”) and multiple solutions for cooperative location have been proposed as alternatives or supplements to satellite location systems. The publication by N. Patwari, J. N. Ash, S. Kyperountas, A. O. Hero, R. L. Moses, and N. S. Correal, “Locating the nodes: cooperative localization in wireless sensor networks,” Signal Processing Magazine, IEEE, vol. 22, Jul. 2005, pp. 54-69 may notably be cited. These location solutions are generally based on measurements of distance between each pair of nodes, and make it possible to estimate the positions, relative or absolute, of each node.
More recently, techniques have been proposed for dealing with the problem of location within the framework of wireless body networks or WBANs.
Thus, in the procedure proposed by C. P. Figueiredo, N. S. Dias, and P. M. Mendes, “3D localization for biomedical wireless sensor networks using a microantenna,” Wireless Technology, 2008. EuWiT 2008. European Conference on, 2008, pp. 45-48, the authors are concerned with the problem of locating electrodes positioned on a body. The proposed procedure is based on the use of low-frequency radio technology, to better cope with the attenuations caused by the body, as well as the use of a positioning algorithm called SPA (for Self Positioning Algorithm). This algorithm is based on the measurements of distance between the various electrodes, obtained by virtue of the metric of the RSSI (for Receive Signal Strength Indicator) and the use of an MEMS (for Micro Electrical Mechanical Systems) micro-antenna of cantilever type. On the basis of the measurements obtained, the SPA algorithm estimates, by means of the least squares procedure, the position of each of the electrodes in a virtual coordinate system. However, this procedure implements exhaustive measurements of distance between each pair of objects. Moreover, the positioning is purely virtual and is limited to applications of the type involving auto-organization and auto-discovery of the network topology. Finally, this procedure is based on the metric (rather inaccurate) of the RSSI.
In C. Guo, J. Wang, R. V. Prasad, and M. Jacobsson, “Improving the Accuracy of Person Localization with Body Area Sensor Networks: An Experimental Study,” Consumer Communications and Networking Conference, 2009. CCNC 2009. 6th IEEE, 2009, pp. 1-5, the authors propose and experimentally evaluate a procedure for locating wireless body networks indoors. Since the antennas are generally not perfectly omnidirectional, the authors propose the idea of distributing several wireless objects over the body, so as to utilize spatial diversity and thus improve the accuracy of location. Measurements of distance are obtained between each object of the wireless body network and exterior apparatus (also called anchors) by virtue of the metric of the RSSI (for Receive Signal Strength Indicator). Since several values of RSSI are obtained for one and the same body network, two positioning procedures are then proposed. The first procedure is based on the calculation of a mean RSSI value for the body network as a whole. The second procedure is based on the calculation of a mean position obtained on the basis of the various positions estimated by each of the wireless objects. However, even though this solution utilizes spatial diversity, by distributing several wireless objects over a person, the cooperation scheme is limited since the distance measurements are performed only between the objects and anchors, and this may lead to insufficient accuracy and/or coverage.
In H. Ren, M. Q. Meng, and L. Xu, “Indoor Patient Position Estimation Using Particle Filtering and Wireless Body Area Networks,” Engineering in Medicine and Biology Society, 2007. EMBS 2007. 29th Annual International Conference of the IEEE, 2007, pp. 2277-2280, the authors propose a technique for positioning body networks or BANs, where each BAN consists of a single wireless node. This technique is based on the RSSI metric and the use of a particle filter which makes it possible to combine: 1) the measurements of RSSI obtained on the basis of anchors or of other BANs within communication range; as well as 2) the corresponding transmission powers, so as to estimate the position of each wireless body network. However, in this procedure, the BANs consist only of a single wireless node, this not making it possible to implement complex cooperation schemes. Finally, the metric used (RSSI) is rather inaccurate.
In C. Lee, H. Lee, and J. Kim, “Performance of a one-way ranging method for WBAN healthcare services,” Communications and Information Technology, 2009. ISCIT 2009. 9th International Symposium on, 2009, pp. 1460-1463, the authors evaluate the performance of a location technique which is based on the metric of the TDOA (for Time Difference Of Arrival) and an exchange of 1-way ranging type between a body network consisting of a single wireless node and a set of anchors whose positions are known. The least squares procedure is then used to estimate the position of each body network. However, this solution does not implement any cooperative schemes during location.
In F. Chiti, R. Fantacci, F. Archetti, E. Messina, and D. Toscani, “An Integrated Communications Framework for Context Aware Continuous Monitoring with Body Sensor Networks”, IEEE Journal on Selected Areas in Communications, vol. 27, No. 4, May 2009, the authors propose a “framework” for the support of medical applications. This system is based on a wireless body network, composed of several objects and a node CH (for Cluster Head), and a network of sensors deployed at the level of the immediate environment. This network of sensors is mainly used by the node CH so as to be able to exchange medical information and to locate itself. Location is performed by virtue of the estimation of the metric DoA (for Direction of Arrival) between the node CH and the nodes of the network of sensors. These measurements of DoA combined with the transmission powers are then used by an algorithm, based on particle filters and dynamic Bayesian networks, for estimating the position of the node CH. However, here again, this technique does not utilize the specifics of body networks. Indeed, only one of the nodes of the BAN (the cluster head) interacts with the infrastructure (networks of sensors in the current case) so as to be able to locate itself. No cooperation scheme of Intra or Inter BANs type is used. Moreover, the distance measurement is based on the RSSI metric and the direction of arrival.
Certain techniques tackle cooperative location in wireless networks and more particularly location, tracking and motion capture of wireless body networks or of nodes placed on a human body.
A first series of techniques is directed mainly at the issue of the cooperative location of mobile or stationary terminals in a wireless network. The American patent application referenced under the publication number US 2008/0268873, the international patent application published under the number WO 2004/095714, European patent application EP 1 207 404, and the American patent applications published under the numbers US 2007/0225016, US 2008/0232281, and US 2002/0055362 may notably be cited. The cooperative schemes proposed are based mainly, on the one hand, on radio communications between the terminals and anchors (or base stations) whose positions are known, and on the other hand, on radio communications between the various terminals.
A second series of techniques is directed more particularly at the issue of the positioning and/or tracking of people indoors and/or outdoors, as well as at motion capture by means of devices placed on a body.
Thus, the techniques presented in the international patent applications published under the numbers WO 2008/143379 and WO 2005/096568 implement non-cooperative tracking and locating procedures. The tracking of a person under surveillance, outdoors, is performed by virtue of a GPS and GSM module disposed on a body. Indoors, the positioning is carried out by virtue of a radio module disposed on a person and which communicates with a base station linked to the local network. However, these techniques do not generally make it possible to obtain accurate location and/or good location coverage.
The technique presented in patent application US 2008/0077326 consists of a system and a procedure allowing the location and the tracking of people operating on sites with risks where GPS is not available. Each person (or BAN) is equipped, inter alia, with a radio module so as to be able to communicate with the base stations or the other people in geographical proximity. Measurements of distance, between the BANs and to the anchors, are performed by virtue of the RSSI metric and make it possible to locate and to track the displacement of people inside the disaster-stricken zone.
The procedure presented in American patent application US 2008/0223131 uses a system based on ultra-sounds to capture the motion of a mobile person or object. A person is equipped with ultra-sound transmitters and receivers, which are themselves linked by cable to a central apparatus. Distance measurements are obtained by virtue of exchanges between the transmitters and receivers. These measurements are subsequently refined with data arising from inertial sensors, thus ultimately making it possible to capture the movements of the person or to detect certain postures. However, this procedure presented undertakes distance measurements in an exhaustive manner, thus requiring a large number of signal transmissions. Moreover, this procedure sometimes turns out to be insufficient in terms of coverage and/or accuracy.
The invention presented in the international patent application published under the number WO 2007/093641 pertains to an autonomous system for determining information representative of the motion of an articulated chain comprising at least two solid elements and at least one articulation linking the said two elements. The system comprises at least two devices for measuring inter-device distances, mounted fixedly on two distinct elements of the said articulated chain and adapted to transmit the measurements performed. Furthermore, the system comprises calculation means, mounted on the said articulated chain, adapted for calculating information representative of the motion of the said articulated chain on the basis of the measurements. However, this system is focused on the direct inter-device distance, the system comprises a constraint in that the devices are placed at very specific spots in order to determine the motion of articulations, and it is limited to motion capture or reproduction applications.
Thus, in the context of wireless body network location applications, the majority of the existing solutions are based on networks composed of a single wireless object as well as on fairly simple cooperation schemes. Indeed, cooperation between the objects of one and the same BAN network (intra-BAN cooperation) is nonexistent, and cooperation between various BAN networks (inter-BAN cooperation) is limited to measurements of distance between two wireless objects, which are generally chosen as being cluster head or coordinator nodes.
Moreover, in the context of applications of auto-organization or motion capture type, the cooperation schemes are based on carrying out exhaustive measurements of distance between each pair of wireless objects of one and the same BAN. Inter-BAN cooperation is furthermore nonexistent and the positioning of these objects is generally of virtual type.
Ultimately, it is apparent after analysis that the majority of the solutions proposed in the context of location, auto-organization or motion capture of wireless body networks, implement only fairly simplistic Inter-BAN cooperation schemes (for example, a measurement of distance between just two nodes) or Intra-BAN cooperation schemes with exhaustive distance measurements without any consideration for the performance aspects of the MAC layer. Furthermore, none of the already existing solutions makes it possible to simultaneously combine various types of Intra-BAN, Inter-BAN and Anchor-BAN measurements.